1. Field of the Invention
The current invention relates generally to optical systems and methods for non-contact examination of objects, and more specifically to optical systems and methods for determining surface profiles or shapes of a test object.
2. Description of the Related Art
The surface shape of an object under test may be obtained through the use of non-intrusive optical diagnostic methodologies. For example, the wavefront produced by light reflected from the surface of a test object may be propagated to a wavefront analyzer such as a Shack-Hartmann wavefront sensor, where the measured shape of the imaged wavefront may be correlated to the surface profile of the object under test. Alternatively, a mask or pattern, such as that produced by a series of Placido rings or Placido-type sources, may be reflected off the test object and re-imaged at a detector. In such systems the surface of the test object may be considered as part of an optical system, so that deviations of the resulting image from an ideal may be used to infer or determine the shape of the test object.
One commercial use of Placido-type sources has been in the measurement of the shape of corneal surfaces of animal and human subjects, for example, in order to provide a custom treatment plan prior to a corneal refractive procedure such as LASIK or PRK. Since a large portion of aberrations produced by an eye typically are generated by the front surface of the cornea, corneal topography can be utilized to provide improved visual outcomes. However, other aberrations can be produced by other portions of the eye, such as the back surface of the cornea, the natural lens, the vitreous humor, a previously implanted intraocular lens, and the like. As a consequence other types of diagnostic instrumentation have been developed such as pachymeters, optical coherence tomography (OCT) sensors, and wavefront sensors, and the like. Such systems may be combined with corneal topographers to provide a more complete analysis of ocular aberrations and to provide treatments resulting in better refractive outcomes. Furthermore, in some systems, the combination of elements can improve the accuracy or fidelity of a given measurement system. For example, the addition of corneal topography information to a system for whole eye wavefront measurement may be useful, not only in understanding the optical system (such as the eye), but in producing better and more accurate information.
In a general sense, Placido-type systems utilize a mapping of points or shapes of a mask or pattern to an image or detector plane in order to deduce what test object shape is responsible for the observed mapping. Such a mapping can become more difficult for complex test object shapes and/or when a highly resolved or high frequency surface features are desired. For example, adjacent points or zones on a topographer mask or pattern may be mapped to very different points or zones in an image or detector plane due to the presence of large curvature gradients on the reflective test object. Corneal surfaces may include such complex forms and thus currently available corneal topographers may have limited accuracy in some cases.
Accordingly, there is a need for measurement systems and methods that are able to provide more accurate surface measurements for relatively complex surfaces, such as those found in corneal topography.